1. Field of the Invention
The present invention relates to a tool center point calibration method for accurately determining appropriate tool center points for a robot to allow for proper tool alignment and correction of robot programs.
2. Prior Art
A tool center point is a six-dimensional vector defining the tool tip position of a robotic tool with respect to a robot face plate. The tool center point is essential so that the robot knows precisely where the tool is located. Although a robot will know the precise location of the robot face plate at any given time, a robot may not precisely know the tool center point for a specific tool due to a variety of reasons. For example, the tool center point for any specific tool of a given tool type will vary between the individual tools of that type. Consequently, pre-specified tool center points are not entirely accurate for any specific individual tool of a tool type. Additionally, the center point may change due to a crash or due to wearing of the tool. The robot, however, requires precise location of the tool center point for appropriate operation. For example, in a spot welding gun, an error in the plane of the workpiece where the robot believes the actual tool center point is will result in a corresponding error in the positioning of the spot welds by the robot on the workpiece.
Consequently, methods of determining the tool center point provide for correcting a robot program if the tool position changes due to a crash, tool change or other extraneous factors.
One known method for determining a tool center point is physically measuring the location and orientation of the tool with respect to the robot face plate and entering the measured tool center point. Subsequent to entering the initially measured tool center point, the gun is swiveled about the position assumed to be the tool center point. If the tip of the tool moves, the assumed center point is adjusted. When the tip of the tool does not move when pivoted about the assumed tool center point, the appropriate tool center point has been achieved. This procedure takes approximately 15 minutes for a skilled technician and has an accuracy of about .+-.8 mm. This particular method requires a skilled technician, a significant amount of downtime for a robot and only offers a minimal amount of accuracy.
Another method for calculating or updating a tool center point for robotic tools utilizes a fixed target or bull's-eye in a precise location. The robot arm is moved to a known target location and the tool is adjusted until the tool is appropriately aligned with the target. This procedure, again, requires a skilled technician and takes approximately 10 minutes. The accuracy of the resulting calculated tool center point is within approximately .+-.2 mm. The difficulty with this solution is that it requires a skilled technician and is time-consuming.
A further prior art method for determining or updating a tool center point involves removing the tool from the robot and moving the robot face plate to a fixed, known position, remounting the tool and taking the tips to the same known position. This removal and remounting of the tool can take 20 minutes and offers an accuracy of .+-.3 mm. Additionally, I have designed a double beam calibration unit specifically for the calibration of arc welding units. My prior art method is described in the article "Robotic Arc Welding in a Flash" in Robotics Today, Vol. 2, Number 4, Fourth Quarter 1989. The difficulties of my prior system are that it requires a pair of orthogonal beams and is limited to calibrating specific types of robotic tools.